1. Field of the Invention
This invention relates to a fire alarm system in which a plurality of fire detectors connected to a central signal station through a pair of signal lines are sequentially called to return detection signals in the mode of current from the detectors for determining a fire based on the returned detection signals.
2. Description of the Prior Art
In conventional fire alarm systems, the signalling between a plurality of fire detectors each including an analog detecting means for detecting, in the form of analog data, a change in the ambient physical phenomena caused by a fire and a central signal station is attained by calling the fire detectors from the signal station by code signals and responding thereto from the fire detectors also by the code signals. The code signals from the signal station comprise pulse codes for the respective fire detectors each of which is composed of an address bit, a response bit and a control bit. The signal station sequentially calls the respective fire detectors by transmitting pulse codes of different address bits and determines a fire if it receives a response any one of the fire detectors.
However, such conventional fire alarm systems are required to provide an expensive coding circuit for each of the fire detectors and they have another problem that if a plurality of types of fire detecting means are employed in the systems and the types of the detecting means are required to be identified, the number of bits required for call and response is increased and it takes considerable time for processing.
In another example of the conventional fire alarm system, the fire detectors are called by code signals and the detectors respond thereto in the form of a change in current amount. In this system, however, the signal station calls the fire detectors by palling and three lines such as a power line, a common line and a signal line are required. The number of the lines may be reduced to two and the signal line may be used in common with the common line. But, the respective detectors are considered to be inherent impedances and a current flows through the signal line or the common line in a normal monitoring condition. Therefore, if an oscillation circuit within the fire detector is energized or deenergized, the value of current is changed and the change is not distinguished from the signal. Thus, there may be caused a misoperation. There is another problem that when the number of the fire detectors is increased, the current flowing through the signal line is also increased. By this reason, there is a limitation in the number of the fire detectors connectable to a single central signal station. There is a further problem that a current consumed by the fire detector is increased when the fire detector is actuated upon detection of a fire and a S/N ratio of a detection signal to be returned to the signal station in the form of a change in a steady-state current is lowered.
As the examples of the relevant inventions known to the inventors, there can be mentioned U.S. Pat. No. 4,287,515 issued Sept. 1, 1981 to Samuel Raber, Allendale; John M. Wynne, Oak Ridge; Alan M. Heinm, Florham Park, all of N.J., U.S. Pat. No. 4,162,489 issued July 24, 1979 to Peer Thilo; Otto W. Moser, Berlin & Munich, Fed. Rep. of Germany, U.S. Pat. No. 4,161,727 issued July 17, 1979 to Peer Thilo; Otto W. Moser, Berlin & Munich, Fed. Rep. of Germany. But, all of these inventions do not solve the problems involved in the conventional fire alarm systems as mentioned above.